There are many situations in which a nut must be secured to a threaded stud and a predetermined tension applied to the stud. (The term "stud" as used herein includes bolts as well as studs that are integrally formed with other structures.) As the nut becomes tighter, the frictional forces increase greatly and in most instances non-linearly. It therefore becomes more and more difficult to determine the stud tension by measuring the torque applied to the nut. A method of overcoming this difficulty is to pull the stud axially, directly applying a predetermined tension, and then turning the nut, applying only a small torque. The stud is then released, but it is held in tension by the nut.
One exemplary environment in which this stud tensioning technique is used is in making up joints in large diameter pipelines. The numerous studs in these joints must be subjected to a repeated pattern of increasing tension, each stud being tensioned in a number of successive operations. If a stud tensioning apparatus is used, it must therefore make a very large number of engagements with the studs before the job is completed. To make up a single joint may take a four or five person crew an entire shift. Not only are labor costs high but the down time may be a much greater cost.
The apparatus commonly used to tension the studs of such a joint includes a nut-like member that must be rotated to engage and disengage each stud, making the apparatus time-consuming to use. The design of such apparatus is limited by the small width of the flange on which the studs are located and the close proximity of the studs to one another. One particularly problematic design consideration arises from the fact that the studs often are not precisely perpendicular to the flange, making proper engagement of the stud threads for tensioning purposes difficult.
Devices have been proposed that would not be threaded onto the studs, but would grasp the threads of the studs by the clamping motion of a jaw. However, these devices have not been found to be satisfactory due to an inability to combine a compact structure with strength, durability, speed and ease of operation.
An objective of the present invention is to provide an improved stud tensioning apparatus that does not rely on rotation to engage the studs and that is capable of meeting the above criteria.